1. Field of the Invention
The present invention relates to a semiconductor diode device having characteristics of slow reverse current reduction, i.e., soft recovery, and to a method for fabricating the same.
2. Description of the Related Art
A diode normally includes an anode region and a cathode region with a junction there between. The cathode region is designed using an N type impurity to contain major carriers (i.e., electrons) and the anode region is designed using a P type impurity to contain major carriers (i.e., holes).
A carrier depletion region may exist on both sides of the junction of the diode device due to an effect where carrier types disappear near the junction due to a recombination process. For example, an electron in an N type impurity layer situated near the junction may be recombined with a hole which has diffused across the junction from a P type impurity region causing the electron to disappear. Similarly an electron may diffuse across the junction and recombine with a hole in the P type impurity layer making the hole disappear from that location. The depletion region which results from this effect consists of depleted electrons on the N side of the junction creating a positive charge and depleted holes on the P side of the junction creating a negative charge.
When a more positive voltage is applied to an anode and a more negative voltage is applied to a cathode, the diode is placed in a forward biased state whereby majority carriers are supplied into both impurity type regions. Near the junction, holes are injected into the N type impurity layer from the P type impurity layer where they become minority carriers, and electrons are injected into the P type impurity layer from the N type impurity layer where they become minority carriers. Majority carriers injected into each impurity layer neutralize some of the charge present in the depletion region causing it to narrow. As the depletion region width narrows, a depletion barrier voltage decreases and majority carriers are allowed to cross the barrier enabling a current flow through the diode.
Considering a reversal of the above applied electrical polarities; when the negative voltage is applied to the anode and the positive voltage is applied to the cathode, the diode is placed in a reverse biased state. Holes are ejected from the P type impurity layer, electrons are ejected from the N type impurity layer, and majority carriers leaving the impurity regions cause the charge present in the depletion region to increase. As the depletion region width increases, a depletion barrier voltage increases and majority carriers are blocked from crossing the barrier disabling a reverse current flow through the diode.
A breakdown voltage of the diode is a reverse bias voltage of a particular magnitude which allows reverse current to flow due to a breakdown mechanism of the diode when a specific threshold value is exceeded. Thus, the diode must be fabricated to have an adequate breakdown voltage for a given application in order to disallow any reverse current flow which may not be desired.
When a forward biased diode device is initially switched to a reverse biased state, a recovery period is needed for the depletion layer to form and effectively block the current. Until the depletion layer is formed, the reverse current will flow in the diode until the depletion layer is wide enough to effectively block the current flow.
A time defined by a period starting at a point where the diode is switched initially from a forward bias state to a reverse bias state, to the time where the current flow is blocked in the reverse biased state is expressed as a recovery rate. The recovery rate of a diode may also be described as a time required for the minority charge removal during a period from which the diode switches from a conducting state to a reverse biased state. The present invention relates to a diode device having the characteristic of a long recovery period (i.e., soft recovery).
A representational cross-section view of a diode having the characteristic of soft recovery fabricated in accordance with a first prior art technique is illustrated in FIG. 1.
The diode includes a shallow P- layer 16 and deeper P-type layer rings 18 for increasing a breakdown voltage of the diode. In the forward biased state, the amount of hole carriers injected into the N type impurity layer is less than the amount of electron carriers injected into the P type impurity layer thus achieving a characteristic of soft recovery. However, the different depths of the P- layer 16 and P-type layer rings 18 increase the number of mask and forming steps in the process, thereby decreasing productivity and increasing semiconductor production costs.
The steps of forming the diode of FIG. 1 include: forming an N- epitaxial layer 14 on an N+ layer 12; forming a shallow P- layer 16 on the surface of the N- epitaxial layer 14; forming P- type impurity rings 18 around the P- layer 16 to adjust the breakdown voltage of the diode; forming an N+ channel stop layer 20 to prevent the lateral extension of the depletion layer; attaching an anode 24 to the shallow P- layer 16; and attaching a cathode 10 to the N+ layer 12.
The anode electrode 24 is attached to the shallow P- layer 16 by selectively opening a contact window and patterning a conductor contact to form the anode electrode 24. The shallow depth of the P layer 16 which may be between 2 .mu.m and 4 .mu.m may result in an unreliable contact base for the anode electrode 24.
FIG. 2 represents an impurity concentration distribution profile taken along the line II-II' of FIG. 1. The impurity concentration profile includes a shallow lightly doped P- region (16), a lightly doped N- region (14) and a heavily doped N+ region (12).
In this first technique according to the prior art, the formation of a shallow P type impurity layer will result in a diode which exhibits increased electron carrier injection into the cathode and decreased hole carrier injection into the anode thereby obtaining the characteristic of soft recovery.
However, the method according to the first prior art technique includes an unreliable step of forming an electrode contact on a shallow impurity layer.
In addition, the method according to the first prior art technique includes additional masks and formation steps for obtaining a desired reverse breakdown voltage.
A method for obtaining the characteristic of soft recovery in the semiconductor diode fabricated in accordance with a second prior art technique uses a deep P type impurity layer to avoid the reliability issues involved with contacting to a shallow impurity region according to the first technique of the prior art.
In addition, separate mask and processing steps for forming P type impurity rings 18 (FIG. 1) are unnecessary when the reverse breakdown voltage characteristic is adequate in, for example, a diode which includes a deep undulating shaped P type impurity layer.
With reference to FIG. 3, the method according to the second prior art technique includes: forming N+ layers 32 spaced apart from each other by a distance on a first major surface of an N- epitaxial layer 34; forming an embedded P- layer 38 in the N- epitaxial layer 34 having an undulating shaped junction with a second major surface of the N- epitaxial layer 34; applying an insulating mask layer (not shown) on the second major surface and patterning for use in selectively forming a P+ layer 36 embedded in the P- layer 38; forming the P+ layer 36 by diffusion within the P- layer 38; forming an insulating layer 40 and an anode 42 attached to the P+ layer 36 and the P- layer 38; and forming a cathode 30 connected to the N+ layer and the N- epitaxial layer 34.
FIG. 4 represents an impurity concentration distribution profile taken along the line IV-IV' of FIG. 3. The impurity concentration profile includes a deep anode region consisting of a heavily doped P+ region (36) and a lightly doped P- region (38), and a cathode region consisting of a lightly doped N- region (34) and a heavily doped N+ region (32).
The technique for forming a diode in accordance with the second prior art technique uses a P layer with a deep undulating shaped junction to avoid the reliability issues involved with contacting to a shallow impurity region encountered in the technique according to a first prior art technique. This second technique also does not require separate mask and processing steps needed for forming P type impurity rings to adjust the reverse breakdown voltage characteristic.
However, since the P- layer 38 junction depth is deeper than that of the diode formed according to the first prior art technique, the hole carrier injection into the N type impurity region during the forward bias state is greater than electron carrier injection into the P type region. Thus a diode formed in accordance with this method has a faster recovery time and a shortcoming exists for extending the recovery time to provide an adequate soft recovery characteristic.
Accordingly, there is a need for a diode exhibiting enhanced characteristics of soft recovery and a method for fabricating a soft recovery diode structure which exhibits such enhanced soft recovery characteristics.